As already known, the combustion engine of a motor-vehicle is cooled by a cooling system that uses as cooling liquid a water/ethylene glycol mixture and has an expansion tank fastened to a support structure arranged in the engine compartment of the motor-vehicle.
The expansion tank is a container communicating with the outside through a series of inlet and outlet ports. These ports are provided at respective sleeves and collars protruding from the container, which are made of plastic material in one piece with the container. During the installation of the cooling system, corresponding ducts are then fitted on such collars to connect the inside of the container with the engine and with the radiator.
In some solutions, at the inlet port, the expansion tank is provided with a cylindrical bushing made of metallic material, generally brass, which has a radial thickness of about 0.5 mm and is inserted in the plastic collar by means of a cold driving process so as to be fastened with an interference fitting to the inner surface of the plastic collar.
The brass bushing has two different functions and objectives: shielding the plastic collar from the heat and the high temperatures of the cooling liquid coming from the engine (in particular from the engine supercharging turbine), and therefore limiting the heat exchange between the cooling liquid and the plastic collar, in order to avoid a rapid deterioration of the latter; and defining a localized stiffening at the inlet port to support the mechanical stresses, in particular due to the vibrations caused by the engine and by the motion of the motor-vehicle.
The aforesaid solution with a brass bushing, although widely used, has some drawbacks. In fact, it has been found that the plastic collar anyway deteriorates in time, despite the presence of the brass bushing.
In particular, because of the vibrations affecting the expansion tank, in time the bushing tends to slip outwards of a few millimetres from the plastic collar where it is inserted. Therefore, the innermost portion of the plastic collar lacks of reinforcement.
This event obviously weakens the plastic collar at its innermost portion. In particular, the vibrations transmitted to the expansion tank increasingly solicit this portion, which therefore tends to have micro-fractures or, even, localized ruptures in the plastic material.
Furthermore, the innermost portion of the plastic collar is no longer protected from the high temperature of the cooling liquid. It follows that, in this area, the plastic material is subject to greater thermal stresses, which therefore cause a further deterioration or in any case a faster deterioration. In particular, severe thermal stresses occur during the engine shutdown phases, since the cooling liquid ceases to circulate in the plant.